In recent years, it has been progressively required to reduce the fuel consumption of vehicles in view of environmental problems, and reduction in size and weight of parts for vehicles has been also severely required more than ever. To satisfy the requirements for reduction in size and weight, research on strengthening of materials and surface strengthening by a surface treatment have been actively performed in the field of compression coil spring parts such as a valve spring used in an engine or a clutch torsion spring used in a clutch, for example. As a result, fatigue resistance and settling resistance, which are important properties in a coil spring, have been improved.
Generally, a process for production of a coil spring is broadly classified as a hot forming method or a cold forming method. The hot forming method is employed in forming a coil spring in which cold forming is difficult due to low workability, such as in a case in which wire diameter d is large, and spring index D/d, which is a ratio of coil average diameter D and wire diameter d, is low. As such a coil spring material, carbon steel and spring steel are mentioned. In the hot forming method, as shown in FIG. 1A, a wire material is heated to high temperature so that the material is easily processed, the wire material is coiled around a core metal so that the material forms the shape of a coil spring, the coil spring is quenched and tempered, and furthermore, shotpeening and setting are performed, so as to obtain fatigue resistance and settling resistance, which are important as properties of a coil spring. In the hot forming method, it should be noted that coiling without using a core metal has not been realized because it is technically very difficult. Therefore, it is necessary to use the core metal in the hot forming method conventionally, and compared to the cold forming method in which coiling is possible without the core metal, a coil spring which can be formed by the hot forming method has a lower degree of freedom in shape.
On the other hand, regarding a coil spring which has a shape which can be formed by the cold forming method due to its comparatively large spring index or comparatively thin wire diameter, the cold forming method has been generally employed from a viewpoint of an easiness of processing technique and a property of mass-production due to processing rate and facility cost (takt time, size accuracy, cost). In addition, as one of the main reasons why the cold forming method is employed, a forming technique without using a core metal is established and the degree of freedom in forming a coil spring is large. In the cold forming method, a hard-drawn wire such as carbon steel wire, hard steel wire, piano wire, and spring steel wire have been used as a spring material. However, strengthening of material has been required from the viewpoint of reducing weight in recent years, and an oil temper wire, which is expensive, is progressively widely used.
In the cold forming method, as shown in FIGS. 1B and 1C, a wire material is coiled to a coil spring shape in a cold condition, it is annealed, and then shotpeening and setting are performed, if necessary. Here, the annealing has a purpose of removing residual stress, which has been generated by a processing and which may become disincentive to improve fatigue resistance of a coil spring. In addition, the annealing contributes to improving fatigue resistance of a coil spring together with giving of compressive residual stress to a surface by shotpeening. It should be noted that a surface hardening treatment such as nitriding treatment is performed before shotpeening, if necessary, in a coil spring which is to be used under high load stress, such as valve spring or clutch torsion spring.
Extensive research to further improve fatigue resistance has been performed. For example, in the Patent Document 1 below, an oil temper wire for cold forming is disclosed, and furthermore, a technique to improve fatigue resistance by using a processing-induced transformation of residual austenite is disclosed. In the Patent Document 2, a technique is disclosed in which multi-step shotpeeing at different projecting rates is performed against a surface of wire material on which nitriding is performed so as to give large compressive residual stress and improved fatigue resistance.
In the Patent Document 1, residual stress is generated in the coil spring after coiling. This residual stress, in particular a tensile residual stress of a wire axis direction that is generated in a surface of an inner diameter side of a coil, is one of the disincentives against improving fatigue resistance as a coil spring. Ordinarily, annealing is performed so as to remove residual stress due to this processing. However, those skilled in the art already know or can easily assume that it is difficult to completely remove this residual stress while maintaining strength of the wire material, even with the wire material disclosed in the Patent Document 1 which has high tempering softening resistance. Therefore, it is difficult to give sufficient compressive residual stress to a wire material surface due to influence of tensile residual stress generated at an inner diameter side of a coil by processing, and therefore sufficient fatigue resistance as a coil spring cannot be obtained, even if shotpeening is performed thereafter. Furthermore, an element such as V or Mo, which contributes to improving tempering softening resistance, is expensive. Therefore, a wire material may be extremely expensive, and a coil spring as a product may also be expensive as an inevitable result.
In addition, the Patent Document 2 discloses that compressive residual stress around the proximity of wire material surface (hereinafter referred to as “surface”) of coil spring is about 1400 MPa, and that this compressive residual stress is sufficient to avoid generating of cracking at the surface as a coil spring used under high load stress in a class of valve spring or clutch torsion spring. However, as a result of improving compressive residual stress of the surface, compressive residual stress inside of the wire material becomes low, and therefore, the effect of the compressive residual stress becomes poor against cracking generation inside of the wire material which starts and spreads from inclusions or the like. That is, by the method of the Patent Document 2, there is a limitation in energy given by shotpeening, although variation can be given to a distribution of compressive residual stress; however, it is difficult to greatly improve the total overall compressive residual stress. In the Patent Document 2, it is not considered to solve the influence of residual stress by the processing mentioned above, and therefore, the effect of improving fatigue strength against wire material having the same strength is poor.
It should be noted that various means for improving surface compressive residual stress have been realized, and as a result, in a coil spring having a wire diameter from about 1.5 to 9.0 mm for example, the maximal value of combined stress which is a sum of applied stress and residual stress by an outer load is in a range of depth from the wire material surface (hereinafter referred to as “depth) of 0.1 to 0.4 mm, and a portion in which the combined stress is greatest is the origin of breaking. Therefore, it is important to keep large compressive residual stress in the range of depth of 0.1 mm to 0.4 mm to obtain fatigue resistance.
Furthermore, as a method in which residual stress generated during coiling process in the cold forming method is almost completely solved and desired strength as the wire material is maintained, a method in which a coil spring after cold forming is heated to an austenite region and then quenched and tempered, can be mentioned. However, in this case, it is difficult to uniformly heat a coil having spring shape in a short time. As a result, irregular structure may be generated, that is, there may be portions having low strength existing, and it may become difficult to obtain desired fatigue resistance, and also difficult to maintain reliability as a product.
Here, a longer heating time is effective as a method to heat uniformly; however, since crystal particles would be coarsened in that case, fatigue resistance may be deteriorated. Furthermore, large processing strain remains in a coil spring after cold forming, and the amount of the remaining processing strain is not uniform in one coil spring, and it may greatly vary among multiple coil springs. As a result, when the processing strain is reduced by heating, the shape may vary greatly and non-uniformly, and the variation in shape among the multiple springs may also be inevitably great. Therefore, in a valve spring or clutch torsion spring or the like for example, it may be difficult to maintain required size accuracy, and it is difficult to apply such reducing means of residual stress by heating, to a mass-production.
The Patent Documents are as follows.    Patent Document 1: Japanese Patent No. 3595901    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2009-226523
As mentioned above, in view of requirements for both further improvement of fatigue resistance under high stress and cost reduction in recent years, a conventional method for production and techniques disclosed in the Patent Documents 1 and 2 and the like are not sufficient solutions and have difficulties. Furthermore, an oil tempered wire, which has been mainly used for cold forming, is expensive, and in particular, an oil tempered wire to which an expensive element such as Ni, V, and Mo has been added is extremely expensive. Furthermore, since residual stress by processing has not been completely solved by annealing treatment after forming, performance of the wire material has not been utilized sufficiently.